1. Field of the Invention
The present invention relates to an anti-skid brake system for an automotive vehicle.
2. Description of the Related Art
Recently, an increasing number of automotive vehicles are loaded with an anti-skid brake system (usually called "ABS system") which is arranged to prevent the wheel from locking at the time of braking. In order to operate the anti-skid brake system, basically, it is required to determine a slip value of the wheel that is incipiently locked and the slip value is determined on the basis of a speed of the vehicle body and a speed of the wheel.
Generally speaking, as disclosed in Japanese Patent Laid-Open Publication (kokai) No. 196,853/1986, a vehicle body speed assumed on the basis of each of the wheel speeds is employed as the vehicle body speed for determining the slip value. Further, Japanese Patent Laid-open Publication (kokai) No. 194,647/1983 discloses an assumed vehicle body speed which can change a parameter for determining the assumed vehicle body speed in accordance with the period of time for which a braking force is controlled so as to be decreased.
On the other hand, in order to appropriately implement the control of the ABS, i.e. the anti-skid brake system, a control value for controlling the ABS is changed so as to agree with a road surface friction coefficient .mu., for example, a threshold value for performing a shift between the control for reducing, retaining and elevating the braking force.
In order to alter the control value for controlling the anti-skid brake system (ABS), it is of great importance to detect an accurate road surface friction coefficient .mu. along the actual road surface on which the automotive vehicle is travelling.
Recently, on the other hand, the control is changed from the control for increasing the braking force to the control for retaining the braking force when the speed of decelerating the wheel reaches a value lower than a predetermined value, and it is proposed that the speed of decelerating the wheel as a threshold value for shifting the braking force is made smaller as the road surface friction coefficient .mu. becomes smaller.
In this case, when the ABS control is performed from the state in which the road surface friction coefficient .mu. is small to the state in which an actual road surface friction coefficient .mu. becomes high, it is of significance to immediately detect the high road surface friction coefficient .mu. with high response in order to ensure a sufficient magnitude of the braking force in agreement with the road surface friction coefficient .mu. and to shorten the braking distance.
More specifically, in instances where a portion of road is lower in road surface friction coefficient .mu., e.g. an ice-covered road surface, than the rest of the road that has a higher road surface friction coefficient .mu., the period of time for which the automotive vehicle passes through the portion of the road having such a low road surface friction coefficient .mu. is shorter than through the rest of the road. However, once it is determined that the road surface friction coefficient .mu. is low, a delay may be caused in a response to determine a shift from the low road surface friction coefficient .mu. to the high road surface friction coefficient .mu. and the braking distance becomes longer due to the delay.